(381) The Interaction of Elective Induction and Estimated Weight on Cesarean Delivery

The optimal management of presumed macrosomia is unknown. The Lancet 2025 "Big Baby" trial found a lower risk of cesarean delivery with induction vs. expectant management. However, it is unknown whether this is meaningfully different than the expected reduction at any birthweight. This study evaluated whether the association between delivery approach (induction vs expectant management) and cesarean risk is modified by birthweight.

Study Design:
Secondary analysis of the ARRIVE trial including nulliparas with singleton, cephalic fetuses randomized to induction or expectant management at 39 weeks. The primary exposure of interest was induction vs expectant management and estimated fetal weight at 39 weeks (approximated from 39-week birthweight or if > 39 weeks, applying the sex-specific birthweight percentile to obtain a 39-week estimate). Estimated fetal weight was analyzed continuously and in quartiles. The primary outcome was cesarean birth. Multivariable logistic regression was performed to analyze the interaction between estimated weight and management strategy on cesarean birth.

Results:

Of 6074 deliveries, the risk of cesarean was significantly higher with increasing estimated fetal weight at 39 weeks: 16.1% in quartile 1 (2353-3053 grams), 17.8% Q2 (2065-3323 g), 19.7% Q3 (3323-3579 g) and 38.0% Q4 (3579-4649 g) (p< 0.01). The odds of cesarean birth with labor induction vs. expectant management varied significantly by estimated weight quartile: Q1 1.02 (0.78 - 1.34), Q2 0.89 (0.69-1.16), Q3 0.73 (0.57-0.94) and Q4 0.69 (0.55 - 0.87). There was a significant interaction (p< 0.01) between management strategy and estimated weight. Figure 1 demonstrates that induction was not associated with a significant reduction in cesarean birth until the estimated weight at 39 weeks was >3245 grams.

Conclusion:

The "ARRIVE effect" of cesarean birth reduction with induction vs. expectant management is only seen with an estimated weight at 39 weeks > 3245 grams. The "dose-dependent" nature of the effect suggests that if resources are limited it may be optimal to target those with a larger estimated fetal weight.